Recently, the demand for a wireless communication service has rapidly risen owing to the generalization of information communication services, the advent of various multimedia services, and the appearance of high-quality services. To actively cope with the demand, a size of a communication system should be raised in the first place. In order to raise a communication size in a wireless communication environment, it is able to consider a method of finding a new available frequency band or a method of raising efficiency for limited resources. For the latter method, a spatial domain for resource utilization is additionally secured to obtain a diversity gain in a manner of providing a plurality of antennas to a transmitter and receiver or a transmission size of capacity is raised in a manner of transmitting data in parallel through each antenna. Such a technology is called a multi-antenna transmitting/receiving technique to which many efforts have been actively made to research and develop.
In the multi-antenna transmitting/receiving technique, a general structure of a multiple-input multiple-output (MIMO) system using OFDM (orthogonal frequency division multiplexing) is explained with reference to FIG. 1 as follows.
In a transmitting end, a channel encoder 101 reduces influence caused by channel or noise in a manner of attaching a redundant bit to a transmission data bit. A mapper 103 transforms data bit information into data symbol information. A serial-to-parallel converter 105 parallelizes a data symbol to carry on a plurality of subcarriers. A multi-antenna encoder 107 transforms a parallelized data symbol into a spatiotemporal signal.
In a receiving end, a multi-antenna decoder 109, a parallel-to-serial converter 111, a demapper 113 and a channel decoder 115 plays functions reverse to those of the multi-antenna encoder 107, the serial-to-parallel converter 105, the mapper 103 and the channel encoder 101 in the transmitting end, respectively.
Various techniques are required for a MIMO-OFDM system to enhance data transmission reliability. As a scheme for increasing a spatial diversity gain, there is space-time code (STC), cyclic delay diversity (CDD) or the like. As a scheme for increasing a signal to noise ratio (SNR), there is beamforming (BF), precoding or the like. In this case, the space-time code or the cyclic delay diversity scheme is normally employed to provide robustness for an open-loop system in which feedback information is not available at the transmitting end due to fast time update of the channel. In other hand, the beamforming or the precoding is normally employed in a closed-loop system in order to maximize a signal to noise ratio by using feedback information which includes a spatial channel property.
As a scheme for increasing a spatial diversity gain and a scheme for increasing a signal to noise ratio among the above-mentioned schemes, cyclic delay diversity and precoding are explained in detail as follows.
First of all, in the cyclic delay scheme, a receiving end obtains a frequency diversity gain in a manner that every antenna transmits a signal differing in delay or size in transmitting an OFDM signal in a system provided with a plurality of transmitting antennas. FIG. 2 shows a configuration of a multi-antenna transmitter using a cyclic diversity scheme.
OFDM symbol is transmitted through each antennas and different value of cyclic delay is applied across the transmit antennas. A cyclic prefix (CP) is attached thereto to prevent inter-channel interference. The corresponding signal is then transmitted to a receiving end. In doing so, a data sequence delivered from a first antenna is intactly transmitted to the receiving end. Yet, data sequences delivered from the other antennas are transmitted in a manner of being cyclically delayed by predetermined bits rather than a previous antenna.
Meanwhile, if the cyclic delay diversity scheme is implemented on a frequency domain, the cyclic delay can be represented as a multiplication of a phase sequence. In particular, referring to FIG. 3, each data sequence on a frequency domain is multiplied by a prescribed phase sequence (phase sequence 1˜phase sequence M) set different for each antenna, fast inverse Fourier transform (IFFT) is performed thereon, and a corresponding result is then transmitted to a receiving end. This is called a phase shift diversity scheme.
The phase shift diversity scheme can artificially introduce frequency selectivity into a flat fading channel by increasing delay spread of the channel at the receiving end. Thereby, a frequency diversity gain or a frequency scheduling gain can be obtained.
The precoding scheme includes a codebook based precoding scheme used for a case that feedback information is finite in a closed loop system or a scheme for quantizing to feed back channel information. The codebook based precoding is a scheme for obtaining a signal to noise ratio (SNR) gain in a manner of feeding back a precoding matrix index already known to transmitting and receiving ends to the transmitting end.
FIG. 4 is a block diagram of transmitting and receiving ends of a multi-antenna system using the codebook based precoding according to a related art.
Referring to FIG. 4, each of transmitting and receiving ends has predefined finite precoding matrixes (P1˜PL). The receiving end feeds back an preferred or optimal precoding matrix index (1) to the transmitting end using channel information. The transmitting end applies a precoding matrix corresponding to the fed-back index to transmission data (xl˜XMt). For reference, Table 1 exemplarily shows a codebook applicable to a case that 3-bit feedback information is used by IEEE 802.16e system supporting a spatial multiplexing rate 2 with two transmitting antennas.
TABLE 1Matrixindex(binary)Column1Column20001  00  10010.7940−0.5801 − j0.1818−0.5801 + j0.1818−0.79400100.7940 0.0576 − j0.6051 0.0576 + j0.6051−0.79400110.7941−0.2978 + j0.5298−0.2978 − j0.5298−0.79411000.7941 0.6038 − j0.0689 0.6038 + j0.0689−0.79411010.3289 0.6614 − j0.6740 0.6614 + j0.6740−0.32891100.5112 0.4754 + j0.7160 0.4754 − j0.7160−0.51121110.3289−0.8779 + j0.3481−0.8779 − j0.3481−0.3289